Arrays

Background

The lay-out of the antenna stations is an important design characteristic of
a synthesis array. A range of antenna array configurations are needed to
cope with the
trade-offs that different observations require between brightness sensitivity,
resolution and observing speed. For an array with a modest number of
antennas, complementary configurations are needed to fill the (u,v) plane.
This was particularly true in the pre-CABB era, when the maximum bandwidth
in each IF band was 128 MHz. The 2 GHz bandwidths provided by CABB can
result in relatively complete (u,v) coverage in a single array configuration
for a continuum source in the cm bands.

East-west arrays

The original ATCA design focused on observations at
centimetre wavelengths and on sources south of
DEC=-25° (where the ATCA had a geographic advantage over northern
hemisphere arrays). The design also focused on full syntheses. With
these objectives and a modest number of antennas, arguably an east-west
array is the best design.

The ATCA has a range of east-west configurations that are well suited to
imaging with complementary arrays from 214m to 6km in length. A 122m
array is also available, giving high brightness sensitivity but many redundant
baselines. It is no longer offered routinely, but can be
requested and will be considered if sufficiently justified.

Hybrid EW and NS arrays

With the development of observing bands at 12mm and 3mm wavelengths, there
was a need to provide arrays that did not require observers to track a
source to low elevation (east-west arrays require a 12 hour observation
to fill the u-v plane). This is achieved by building
a 214m north spur. This allows so-called hybrid arrays, which
combine antennas on the east-west and north-south arms of the array. These
arrays also allow the ATCA to image at more northern declinations, with
reasonable u-v coverage still achieveable at DEC=+20°.

Hybrid arrays also allow much greater brightness sensitivity observations
(antennas can be packed closer together in two dimensions than in one),
and can have less shadowing than east-west arrays (see the
shadowing diagrams). Three hybrid
arrays are regularly scheduled for the ATCA with baselines up to about 250m.

A single snapshot observation with the hybrid arrays can provide 2D positions
and structures for simple sources, whereas at least two and preferably more than
three separate hour angles are needed with the EW arrays alone.

A hybrid array

Special arrays

On request to the Time Allocation Committee, it is possible to schedule
observer-designed special array configurations. These proposals are viewed
more favourably if they can justify at least several days of observations,
preferably 24 hours per day.
Original: Bob Sault (08-Sep-2005)
Modified: Bob Sault (09-Sep-2005)